possible interfering substances such as dissolved low molecular weight polyethylene or certain additives which, therefore, d o not have to be separated from the BHT or other electron capturing compounds. The problem of separating the additive from the solvent peak to eliminate measuring the BHT on the solvent tail is overcome by using a nonelectron capturing solvent. Thus, using the electron canture detector, a 1-pl injection of hexane showed a sharp solvent peak with a rapid return t o the original baseline. Roberts (6) solved this problem for the flame ionization detector by using carbon disulfide which has a low sensitivity to the flame but would not be satisfactory for the electron capture detector. The use of the flame detector was not necessary in our work as we were only measuring BHT. It was an aid in that it showed when the solvent had cleared the column and another
sample could be injected. We have continued to use both detectors t o expand our work to other additives which may or may not be electron capturing. This method, with a few modifications, could be extended to include many other antioxidants or additives used in other polymers or copolymers. By using a dual detector gas chromatograph (flame ionhation and electron capture), noncapturing compounds could be determined simultaneously. The above method has been used successfully in analyzing polyethylene samples containing 0.5 to 1000 ppm BHT. RECEIVED for review May 11,1967. Accepted August 4,1967. Research carried out as a part of a study on the Phosphorometric Analysis of Drugs in Blood and Urine supported by a grant from the U. s. Public Health Service (GM-11373-04). ~
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A Statistical Method for Evaluation of Limiting Detectable Sample Concentrations P. A. St. John,’ W. J. McCarthy,2 and J. D. Winefordner Department of Chemistry, Unicersity of Florida, Gainesdle, Flu. 32601
THELIMITING DETECTABLEsample concentration, C,, for any instrumental technique, in which readout noise is white, Le., random, and readout drift is absent, is usually, e.g., in spectrochemical methods (1-7), defined as that sample concentration, C,n, resulting in a readout signal-to-rms noise ratio, SIN, of
confidence level, and for any number of sample-background pair observations can be derived as follows. The variance of the difference, Vc-g, between the sample mean, and the background mean, B , in any instrumental experiment is defined ( 8 ) as
u,
vc-B = S2C-B where 3 corresponds to three standard deviations, l o 3 to
